An article of footwear may be constructed with a foam sole portion. The formation of the foam portion may involve forming the foam portion into a pre-form foam portion having a size, shape, or surface that is not intended for a final assembly into an article of footwear. The forming of the pre-form foam portion may result in a higher density surface portion, such as a skin, being formed on the pre-form foam portion. The skin on the pre-form may increase the effort of further processing of the pre-form foam portion to result in a desired subsequent or final foam form that is intended to be assembled into an article of footwear.